Energy Transfer as Heat

S8P2 d. Describe how heat can be transferred through matter by collisions of atoms (conduction) or through space (radiation). In a liquid or gas, currents will facilitate the transfer of heat (convection).

Background All forms of matter, whether solid, liquid, or gas, are composed of atoms or molecules in constant motion Because of this constant motion, all atoms have thermal (heat) energy Whenever a substance is heated, the atoms move faster and faster When a substance is cooled, the atoms move slower and slower The “average motion” of the atoms that we sense is what we call temperature Temperature and heat ARE NOT technically the same thing Temperature is the average motion of atoms and molecules; heat is the energy that flows due to temperature differences

Lesson Essential Question: How is energy transferred as heat?

The transfer of energy between the particles of two objects due to a temperature difference between the two objects is called heat.

The transfer of energy always takes place from a substance at a higher temperature to a substance at a lower temperature Temperature is an indicator of the particles’ average kinetic energy; therefore, you can use it to predict which way energy will be transferred The greater the difference in the temperatures of two objects, the more energy that will transfer as heat

Energy can be transferred as heat in three ways: Conduction Convection Radiation

Conduction

Conduction Conduction is the transfer of energy that occurs when molecules bump into one another (heat transfer from one atom to another within a substance).

Solid at Different Temperatures Conduction Molecules are always in motion, but molecules in warmer objects move faster than molecules in cooler objects. Solid at Different Temperatures

Conduction When objects are in contact, energy is transferred from warmer objects to cooler objects.

Conduction Example of conduction: A spoon in a cup of hot soup becomes warmer because the heat from the soup is conducted along the spoon Conduction is most effective in solids, but it can happen in fluids. Gases and liquids are generally poor conductors of heat because their molecules are farther apart than are the molecules in solids. Therefore, neighboring molecules in a gas or in a liquid are less affected by the increased motions of heated molecules, and consequently heat is not conducted rapidly.

Conduction When you heat a metal strip at one end, the heat travels to the other end. As you heat the metal, the particles vibrate, these vibrations make the adjacent particles vibrate, and so on and so on, the vibrations are passed along the metal and so is the heat. We call this? Conduction

Conduction Why does the lady in the pink dress drop her roasting stick when the lady in the blue dress does not?

When heat is conducted, it is led from the heat source by traveling along a physical object. The young woman in the pink dress was using a straightened out coat hanger wire to cook her marshmallow. Metal is a good conductor of heat. The heat from the fire traveled along the thick metal wire. The wire got so hot that the girl had to let go of it. The young girl in the blue dress is using a wooden rod to hold her marshmallow. Wood does not conduct heat well. The wood in her hand is still cool and the marshmallows are almost done!

Conduction Any material through which energy can be easily transferred as heat is called a conductor What determines how well a substance conducts is whether it is a gas, liquid, or solid Gases are extremely poor conductors because their particles are far apart Liquids conduct better than gases, but they are still not very effective conductors Some solids, like rubber and wood, conduct energy about as well as liquids Metals such as copper and silver conduct heat transfer very well Some solids conduct better than others In general, metals are better conductors than nonmetals

Conduction A material that is a poor conductor that is used to reduce or stop energy is called an insulator Ex. Wood

Conductivity Chart

Convection

Convection Convection is the transfer of heat by the flow of material. Convection circulates heat.

Convection Heat moves with the fluid Warmer portions are less dense and therefore rise; cooler portions fall because they are denser. When the atoms at the top cool down, they become more dense and fall and vice versa for the warmer atoms. Hence a continuous cycle The cycle of a heated fluid that rises and then cools and falls is called a convection current

Fluid movement Cooler, more d____, fluids sink through w_____, less dense fluids. ense armer In effect warmer liquids and gases r___ up. ise Cooler liquids and gases s___. ink

Convection occurs mostly in gas or liquid.

Convection

Water movement Cools at the surface Convection current Hot water rises Cooler water sinks

Radiation

Radiation Radiation is energy that is transferred in the form of rays or electromagnetic waves. This includes infrared radiation, visible light, and ultraviolet rays.

Radiation What happens in space? There are no solids touching the sun for conduction to occur There are no fluids in space for convection to occur Therefore, heat travels by radiation

Conduction, Convection, and Radiation together distribute the Sun’s heat throughout the Earth